In coating of ferrous metals, such as in galvanizing, parts to be coated are immersed into a bath coating material after having been chemically pretreated and cleaned. The amount of time the parts stay immersed depends upon the material of the parts, their shapes, the bath temperature, the coating composition, and the desired coating thickness.
Coating is frequently used to coat continuous strips of ferrous base metal to produce iron or steel strip stock having a thin coating of zinc, aluminum, or the like. In continuous-strip coating, the strip to be coated is first cleaned and pretreated, passed through a bath of molten coating material, and then withdrawn from the bath in a generally upward direction. The coating material adhering to the withdrawn strip is finished by coating rolls, air knives, or the like, and is subsequently solidified.
The molten coating material, usually a molten metal such as zinc, for example, is contained in an externally-heated iron or steel pot. Metal coating pots have several disadvantages, however. They have a relatively short life. This is due to several factors, including rapid build-up of dross at the bottom of the pot.
In normal operation, the coating pot is kept full with molten metal supplied via a launder from a premelter. Where a bottom dross generating alloy, such as a zinc alloy, is used, it becomes necessary to periodically empty the pot completely, and allow it to cool, to remove the dross which has collected at the bottom of the pot. To avoid this, it has been proposed to remove dross by using a circulating pump to continuously pump molten metal through a filter, to remove the dross, and return it to the coating pot. Zinc filtration is not yet a proven technology, however, and there is a need to provide an effective yet simple method of dross removal. The present invention fills that need.